1. Field of the Invention
The present invention relates to a process for producing a metal oxide-type superconductive material. More particularly, the present invention relates to a process for producing a metal oxide-type superconductive material which is suitable for use in various products utilizing superconductivity because said superconductive material has a high critical current density. The metal oxide-type superconductive material obtained by the present invention is used as a material for superconductive magnet, superconductive generator, electric storage system, electric transmission system, Josephson device, sensor device, small-sized generator, small-sized cell or battery, etc.
2. Description of the Prior Art
Metal oxide-type superconductive materials are a new high temperature superconductive material which was first reported in April, 1986 by Zurich Research Institute of IBM. Since then, vigorous researches have been made on these superconductive materials in various countries. Rare earth metal-containing metal oxides represented by Y-Ba-Cu-O systems are drawing special attention as a high temperature superconductive material having a critical transition temperature (Tc) of 77 K (a boiling point of nitrogen) or higher. Further, it has been known recently that Bi-Sr-Ca-Cu-O systems and Tl-Ca-Ba-Cu-O systems both containing no rare earth element give similar Tc's.
These superconductive materials of rare earth element-containing metal oxide type are ordinarily obtained by a process comprising synthesizing metal oxide powders by a wet or dry method, sintering the pressurized powders to obtain a molded sintered material, and heat-treating the material at about 900.degree.-950.degree. C. followed by slow cooling.
There is also known a process for producing a metal oxide which comprises compounding metals as starting materials, melting them to obtain an alloy, and subjecting the alloy to an oxidation treatment.
In the former process wherein the metal oxide powders produced from synthesis by a wet or dry method are sintered, the resulting metal oxide-type superconductive material is inevitably a sintered metal oxide material and accordingly has problems of, for example, (1) having a low strength and being very fragile, (2) having a low packing density and containing a lot of pores and accoridngly causing deterioration of properties in the air, and (3) having a low critical current density (a low Jc).
In the latter process wherein a molten alloy is oxidized, many rare earth elements such as Y, Er, Gd and the like tend to cause phase separation when melted with Ba and are difficult to form a uniform mixture with Ba; therefore, the process is applicable only to the production of a Eu-Ba-Cu-O system or an Yb-Ba-Cu-O system wherein the raw material metals are easily miscible with each other when melted.
The present invention has been made in order to solve the above mentioned problems of the prior art.
An object of the present invention is to provide a novel process for producing a novel metal oxide-type superconductive material, which enables (1) the use of rare earth elements other than Eu and Yb, (2) the obtainment of a superconductive material having a low porosity and a high density, and (3) the obtainment of a superconductive material having a high Jc.
Other objects of the present invention will be apparent from the following description.